This procedure is defined in the server but not documented via ad_proc or proc_doc and may be intended as a private interface.
The procedure is defined as:
proc dom::Encode {value} { array set Entity { $ $ < < > > & & \" " ' ' } regsub -all -- {([$<>&"'])} $value {$Entity(\1)} value return [subst -nocommand -nobackslash $value] }